From a practical point of view, 10 per cent IM distortion about 2 per cent harmonic is tolerable for brief periods; and of course peak pas- sages are of short duration. A maximum of 1 per cent harmonic distortion will require 3- or 4-db sacrifice in signal-to-noise ratio, while a 2- or 3-db increase in signal to noise ratio may be had by accepting 3 per cent harmonic distortion as a satis- factory maximum, which some tape re- corders do. The NARTB standard for measuring signal to noise ratio is based on 2 per cent harmonic distortion and specifies 15 i www.
Record- playback fre- quency response at both speeds with the amplifier described. The writers' amplifier, based on a playback level equivalent to that containing 2 per cent harmonic distor- tion, has a signal-to-noise ratio at cps of approximately 55 db.
However, the noise measurement included frequen- cies below 50 cps and into the sub-audi- hle range. Quite likely, had frequencies below 50 cps been excluded in keeping with the NARTB standard, the measured signal to noise ratio would have appre- ciably exceeded 55 db. Measurement of the amplifier's signal- to-noise ratio was made in the playback mode with the grid of the input tube shorted out, so that noise contributed by the tape and hum contributed by the playback head were excluded. The effect of the last two factors is discussed in the next section. The signal-to-noise ratio of about 55 db is based on the maximum output containing 2 per cent harmonic distor- tion available at 7.
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There is no reason to believe that output from this head is ap- preciably higher or lower than from the general run of heads encountered in home tape recorders. If a full-track head were used, about 6 to 8 db more signal could be expected, and accordingly the amplifier's signal-to-noise ratio could be rated this much higher.
A signal-to-noise ratio of 55 db may seem low in comparison with ratios of 70, 80, or 90 db found in other audio equipment. However, the signal pro- duced by the play head is relatively low ; maximum output from the head em- ployed is perhaps 10 millivolts. By com- parison, peak output from magnetic cartridges in common use may range from about 30 millivolts to as high as Here, then, is a difference of at least 10 db. Furthermore, a tape ampl- fier that conforms to the NARTB play- back curve, as does this one, has a fairly enormous amount of bass boost, so that hum becomes the principal limiting fac- tor.
Between the first turnover point of 3, cps, where playback boost begins to take effect, and the second turnover point of 50 cps, where boost begins to level off, there is a rise of about 30 db. In the case of a phonograph record con- forming to NARTB standards, only 15 db of bass boost is required between the turnover points of and 50 cps. In comparison with other program sources, a tape recorder having a 55 db signal-to-noise ratio appears satisfac- tory.
For disc recordings, the ratio gen- erally varies from 45 to 55 db, based on maximum signal level. In the case of FM, the transmitted program may achieve a signal-to-noise ratio of about 55 db based on full modulation; the bet- ter FM receivers also attain a ratio of about 55 db on signals of normal strength. Top-quality professional machines sel- dom attain a signal to noise ratio much beyond 55 db for a half-track recording made at 7. Under the best of conditions, which includes careful adjustment, use of full- track heads, and operation at 15 or 30 ips, the signal-to-noise ratio which can be expected from the finest commercial tape recorders does not appreciably ex- ceed 60 db.
On the basis of these indica- tions as to the present state of the art, an amplifier with a potential 55 db sig- nal to noise ratio based on the output available from a half-track head may be considered to meet today's standards of high fidelity. Over-all Signal-to-Noise Ratio The reference here is to the signal- to-noise ratio of the entire tape recorder, taking into consideration hum picked up by the playback head and tape hiss, as well as noise and hum generated in the amplifier.
The writers' method of determining the over-all ratio is as follows. A cps signal is recorded on virgin bulk- erased tape at a level which produces 2 per cent harmonic distortion.
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The tape is played back and output measured. The tape is bulk-erased and again sub- jected to the recording process, but this time with no audio input and the volume control all the way down. Again the taoe is played back and output measured. This time the output consists entirely of noise and hum : amplifier noise in re- cording tube noise and hum, resistor noise and noise resulting from distortion in the bias-frequency waveform ; am- plifier noise and hum in playback; tape hiss; and hum picked up by the play- back head.
The signal-to-noise ratio is the ratio between the first and second measurements. This procedure, applied to the writers' amplifier in conjunction with the partic- ular transport mechanism and record- playback head used, indicated an over-all signal to noise ratio of approximately 50 db. The above procedure departs from NARTB specifications in that a bulk eraser is used instead of the erase head because the former does a somewhat bet- ter job.
It was considered inappropri- ate to allow the erase head to set a limi- tation on the signal-to-noise ratio that can be obtained from the machine in view of the ready availability of bulk erasers. Investigation with an oscilloscope re- vealed that the dominant component of noise and hum was hum picked up by the playback head. The particular head used Dynamu incorporates a mu- metal shield that affords considerable protection against hum.
However, this was not enough. For completely effective shielding, as found on professional and semi-professional transports, there is re- quired a mu-metal cover which corn- Fig. Modified Pentron tape-transport mechanism with the new amplifier mounted in place. In order to attain an over-all signal- to-noise ratio as high as 50 db without the benefit of a mu-metal cover, the writers improvised a shielding "gim- mick," described in the section on com- batting hum, which reduced hum pickup about 6 db.
But a transport with better shielding for the head would come sig- nificantly closer to the potential 55 db signal-to-noise ratio made possible by the amplifier. Frequency Response Figure 1 shows frequency response at 7. Response is 3 db down at 30 and 12, cps. Considering re- sponse 6 db down as the limit of the use- ful range, this range extends from 20 to 14, cps.
It would have been quite easy, with the head employed, to extend high-end response so that it would not be more than 2 or 3 db down at 15, cps. However, as explained later, this could be achieved only at the expense of a lower signal-to-noise ratio or an in- crease in distortion or a combination of the two. In the writers' opinion, a loss in frequency response beyond 12, cps is of less consequence than a reduction in signal-to-noise ratio or an increase in distortion.
They feel that response faith- ful to 12, cps or so is virtually indis- tinguishable from response flat to 15, cps. In short, there are no marked peaks that will appreciably color reproduction.
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This was confirmed by toping a high-quality disc, playing back the tape in synchronization with the record, and switching rapidly between the two. Possibly to an extremely sensitive ear the 2-db peak in the region of cps might be discernible on a painstaking A-B comparison in otherwise dead silence. However, under practical listen- ing conditions it would go unnoticed.
By turning his head or moving to an- other seat in the room, the listener will be subjected to far greater changes in frequency response than are caused by the slight departure of the tape recorder from perfectly flat response. It may be mentioned here that the slight rise in bass response around eps is due to the head rather than the amplifier; this is explained in the section on Circuit Details.
Figure 1 also shows frequency per- formance at 3.
Response is down 3 db at 20 and cps, while the useful range at the high end extends to about 7, cps. Certainly this contradicts the oft-made statement that 3. While re- sponse good to or cps is not commensurate with the exacting stand- ards of high fidelity, it can still provide pleasurable and fairly accurate repro- duction of music and other sound. This bandwidth exceeds that of most AM broadcast receivers although some AM stations go out to 10, cps or higher , yet no one has seriously tried to write them off as a source of enjoyable sound. Moreover, the bandwidth at 3.
Response at 3. In view of the limited high-frequency response at the slow speed, it becomes all the more de- sirable to avoid peaks in the bass region. Although bass response could have been smoothed out, this was not done inas- much as primary attention was given to the requirements for good reproduction at 7. All in all, however, reproduction at 3.
Signal Levels On radio input — signal from a radio, TV, phonograph preamplifier, audio con- trol unit, etc. Inasmuch as the signal sources just indicated usually produce 0. On microphone input, 3 mv is required to drive the amplifier to maximum re- cording level. This is sufficient sensitivity to accommodate most dynamic micro- phones having relatively low output, such as those rated in the neighborhood of - 55 db below one volt per microbar.
With such a microphone, the human voice at normal level produces about 2 mv at a distance of two feet, and of course peaks or loud tones produce an output well in excess of that required to drive the recorder. Inasmuch as crys- tal microphones generally have higher output than dynamic ones, there is no problem of sufficient sensitivity with re- spect to the former. The maximum permissible recorded signal at cps produces an output of 0.speccoporsupab.ga
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This is enough to drive virtually any audio control unit or control-ampli- fier combination to full output. In fact, most modern components of this kind can be driven to full output by signals ranging from 0. Amplifier Distortion In recording from the radio input, high level signals can be accommodated without overload inasmuch as the volume control is directly across this input.
When the control is set to a position which corresponds to maximum permissi- ble recording level, IM distortion con- tributed by the record amplifier is ap- proximately 0.